The adolescent brain is driven to seek out novel reinforcers, but the areas of the brain involved in impulse control and executive function remain immature until early adulthood. This may make adolescents particularly susceptible to overeating and weight gain. Motivation to eat, or reinforcing value, is a strong predictor of energy consumption and adiposity. Understanding factors that lead to higher levels of the reinforcing value of food is a critical first step toward developing novel treatment and prevention strategies for obesity. Our previous work has shown that repeated intake of snack foods, such as potato chips and chocolate candy, decreases the reinforcing value of that snack food in non-obese adults, but increases the reinforcing value of that snack food in a subset of obese adults. To date, this relationship has been demonstrated in cross-sectional studies. In addition, our preliminary data, presented in this application, suggest that individuals who increase responses for healthier snack foods (energy density <1.5 kcal/gram) weigh less than those who decrease responses for these foods after repeated administration. This suggests that sensitization of the reinforcing value of food may be a risk or a protective factor against obesity depending on the type of food being consumed. Another protective factor may be having a high reinforcing value of physical activity. Studies have shown that individuals who find physical activity very reinforcing weigh less. Finally, our work and that of others suggests that individuals who are high in reinforcing value of food and high in delay discounting may be at greater risk for weight gain. The purpose of the proposed study is to 1) determine if increases in the reinforcing value of high energy density snack foods predict weight gain over time. 2) Determine if increases in the reinforcing value of low energy density snacks protects against weight gain over time 3) to determine if reinforcing value of physical activity and/or delay discounting moderate the relationship between sensitization of the reinforcing value of food on weight change in adolescents. These studies will allow us not only to determine the weight-related consequences of sensitization of the reinforcing value of food, but also to identify factors that moderate changes in body weight over time as they relate to the reinforcing value of food. Once these factors are identified and characterized, future studies could focus on refining methods for altering the reinforcing value of food as a way to increase healthy eating and decrease less healthy food intake.